A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light. The active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and passage of an electrical current through the p-n junction of a semiconductor laser. Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices are also referred to as laser diodes. The efficiency of such lasers in converting electrical power to optical output power is relatively high, and for example, can be in excess of 25 percent.
In order to effect efficient optical pumping, the photons from a pump source such as a Nd:YAG laser, to the lasant material must have a wavelength in a very narrow range.
It is not uncommon for laser diodes to have lifetimes in excess of 50,000 hours. However, certain factors adversely effecting the lifetimes of laser diodes include high device temperature, current spikes and imperfect output facets.
A laser diode can be fabricated by cleaving a crystalline semiconductor material on which appropriate epitaxial layers are deposited along at least two crystal planes thereby forming an optical cavity, with a pair of oppositely placed facets or mirrors. One of the facets can be coated with a highly reflective coating, and the other with a partially reflective coating. If sufficient current flows through the p-n junction formed by the epitaxial layers or if the structure is optically pumped, lasing occurs and laser light will escape through the partially reflective coated facet.
Device failure due to imperfect output facets can result in degradation instantly or gradually. It is caused by striations and microcracks on the output facet of lasers, thermal effects and the combination of the above in conjunction with thermal oxidation.
Striations, as used herein, are undesirable deviations of a cleavedplane from an ideal flat surface coinciding with a crystal plane. During the cleaving step, undesirable striations are generated when the cleaved plane does not coincide with a single plane. Striations can adversely affect device yield and result in more rapid degradation of optical radiation sources, thus reducing their lifetime.
Over the years a number of methods for producing separate semiconductor components from a semiconductor crystal body have been suggested, such as U.S. Pat. Nos. 3,542,266, 2,970,730 and 4,237,601. These methods have met with varying degrees of success.
U.S. Pat. No. 4,237,601 is directed to a method of cleaving a semiconductor wafer into individual devices, said wafer comprising a substrate, at least a portion of one surface of which is metallized, and a plurality of semiconductor layers deposited on at least a portion of the opposite surface, at least one of which layers, when appropriately biased, generates coherent electromagnetic radiation, which method includes: (a) forming channels of substantially parallel sidewalls about 1 to 4 mils deep in the substrate, said substrate being at least about 6 mils thick; (b) etching into the substrate with an anisotropic etchant to a depth sufficient to form V-grooves in the bottom of the channels, said V-grooves terminating at a point before reaching the said one layer; and (c) mechanically cleaving the wafer including the said one layer along the etched grooves to form bars of diodes. The examples illustrate a high density of striations and damage to the lasing facets along the plane of cleaving. Thus, there remains a need in the art for a method of cleaving semiconductor crystal bodies, resulting in increased device yields and reduced striations on lasing facets, to provide a substantial improvement over prior art techniques.
It is therefore desirable to provide an improved method for cleaving a semiconductor crystal body, which overcomes most if not all of the aforementioned problems.